Conventionally, power semiconductor elements made of silicon (Si) have been widely used. To save energy, power semiconductor elements have recently required improvements in property. Accordingly, power semiconductor elements made of silicon carbide (SiC) have begun to be used as next-generation power semiconductor elements that are resistant to high voltage and involve small loss. Examples of a power semiconductor element include a Field-Effect-Transistor (FET) having a Metal-Oxide-Semiconductor (MOS) structure, and a Schottky diode. A SiC-MOSFET can have a structure in conformance with a basic element-structure of a Si-MOSFET. Although the SiC-MOSFET and the Si-MOSFET have such a common structure, the SiC-MOSFET can operate at a higher temperature than the Si-MOSFET because SiC has a wider bandgap than Si. To be specific, the SiC-MOSFET can operate at 200° C. or more, whereas the Si-MOSFET commonly operates at less than 200° C. As described above, the SiC power semiconductor element can operate at higher temperature than the Si power semiconductor element. In addition, SiC achieves advantages such as smaller loss and higher-speed operation.
Wire bonding is used as a typical method for electrical connection between a power semiconductor element and an external circuit. That is, a conductor wire is joined to an electrode disposed on a surface of the element. Although a conventional and widely-used conductor wire is an aluminum (Al) wire, a recent study provides a copper (Cu) wire. Cu has a higher conductivity than Al. Hence, a Cu wire has a reduced electrical resistance. Moreover, Cu has higher yield strength than Al. Hence, a Cu wire has enhanced reliability against temperature cycles.
A Cu wire has a greater impact on a site where the wire is joined than an Al wire. In particular, power semiconductor elements control large current; therefore, they require a large-diameter wire, which has a greater impact in wire bonding.
For one thing, Japanese Patent Application Laid-Open No. 2013-243166 (Patent Document 1) discloses a technique of joining a copper wire to an electrode of a SiC power semiconductor element. The electrode has a titanium layer and an aluminum layer. The copper wire is joined to the aluminum layer along with ultrasonic vibration. This publication describes that the titanium layer, which is a hard material, reduces damage to the electrode of the power semiconductor element. Around the electrode, a polyimide layer is disposed so as to be in contact with the edge of the electrode. The polyimide layer serves as a protective film around the electrode. The protective film enhances the reliability of the power semiconductor element.
For another thing, Japanese Patent Application Laid-Open No. 2014-082367 (Patent Document 2) discloses joining a wire made of Cu or an alloy of Cu (hereinafter simply referred to as a “copper wire” or “Cu wire”) to an electrode on a SiC monocrystalline chip. The electrode includes a top layer made of Cu or an alloy of Cu, and a protective layer with high hardness. This publication describes that the protective layer prevents cracking in the chip in wire bonding. Moreover, the wire and the top layer, both of which are made of Cu or an alloy of Cu, are well joined to each other.